Platinum group elements include noble metal elements such as ruthenium, rhodium, palladium, osmium, iridium and platinum, in which the elements ruthenium (Ru), rhodium (Rh), palladium (Pd) and platinum (Pt) have an excellent catalytic performance, and have been widely used in the petrochemical industry and various reactions in organic chemistry and are excellent catalysts or one of the important components of the catalysts. However, when the noble metal catalyst is prepared using a conventional impregnation method, under the influence of the surface tension of the impregnating solution and the solvation effect in the drying stage, the precursor of the noble metal active component is deposited on the surface of the carrier in the form of aggregates; such a high degree of aggregation is difficult to break down in the subsequent calcination process, which seriously affects the dispersion degree and catalytic activity of the active component. Additionally, when the supported metal catalyst is prepared using a conventional impregnation method, calcination and reduction processes must be carried out to obtain a zero-valent noble metal catalyst having catalytic activity, and the preparation process is therefore complicated.
Hydrotalcite is a type of anionic layered compound, a member of a class of materials also known as layered double hydroxides, abbreviated as LDHs. It has characteristics such as a regular arrangement of the interlayer anions and the chemical composition of slabs, a weak alkalinity, and good thermal stability, and can be used as a carrier for a noble metal catalyst. The composite metal oxide produced by calcination of a hydrotalcite precursor has a high specific surface area and good thermal stability, and a noble metal catalyst having a high degree of dispersion with the composite metal oxide as the carrier can be obtained by subjecting the hydrotalcite-supported active component to calcination and reduction. However, the aforementioned catalysts prepared by conventional methods such as impregnation must go through the steps of calcination and reduction before a form of the noble metal element having catalytic activity can be obtained.
Alumina has characteristics such as a large specific surface area, good thermal stability, high mechanical strength, an adjustable surface acidity and a low price, and has been widely used in the fields of petrochemical industry and catalysis. Currently, the catalyst carrier used in hydrogenation processes is generally alumina in both China and elsewhere. In terms of the geometric shape, alumina carriers can be divided into spherical, bars, cylindrical, trefoil, honeycomb shape and the like. Compared with the others, spherical carrier particles, which are in contact with each other at points in a fixed bed, bear stress evenly, easily give a filling of a dense phase, and are not prone to channeling, offer improved mass transfer and catalytic performance, and have become a focus of research in recent years.
In Document [1] Effects of Pd precursors on the catalytic activity and deactivation of silica-supported Pd catalysts in liquid Phase hydrogenation, Applied Catalysis A, 2005, 292:322-327, catalysts with SiO2/MCM-41 as the carrier are prepared using different Pd precursors, Pd(NO3)2, PdCl2 and Pd(OOCCH3)2 by the impregnation method, and the experimental results show that the Pd particles in the prepared catalysts have a small size, a high degree of dispersion, and a high catalytic activity in liquid-phase hydrogenation reactions. However, after five cycles of reactions, the active ingredient—Pd particles—become severely sintered, and their catalytic activity is decreased significantly.
In Document [2] Hydrogenation of 2-butyne-1,4-diol on supported Pd catalysts obtained from LDH precursors, Microporous and Mesoporous Materials, 2007, 99:118-125, Pd catalysts are prepared using hydrotalcite as the precursor by the impregnation method, coprecipitation method, and ion-exchange method, and the test results show that the catalysts prepared by these three methods have a large specific surface area, a high degree of dispersion of the active ingredient, and a good catalytic activity and a good selectivity. However, similarly to the case for other supported metal catalysts, calcination and reduction steps must be carried out to obtain Pd0 having catalytic activity, and the preparation process is complicated.
In Document [3] Selective hydrogenation of acetylene over Pd catalysts supported on nanocrystalline α-Al2O3 and Zn-modified α-Al2O3, Catalysis Communications, 2008, 9:2297-2302, Pd—Al2O3 catalysts are prepared using Al2O3 as the carrier by the sol-gel method and the solvothermal method respectively, and the characterization results demonstrate that the metallic Pd in the catalysts has a large particle size and a low degree of dispersion.
In summary, the preparation methods currently used for supported noble metal catalysts are complicated, and calcination and reduction steps are required after the noble metal has been supported; during the calcination, the particles of the active noble metal component easily become sintered, which has a serious adverse effect on the catalytic activity. Therefore, the development a noble metal catalyst produced by a simple preparation method, having a large specific surface area of the carrier, a high degree of dispersion of the metal and a high utilization ratio of the active component, a good catalytic activity and a good selectivity has become an important target.